U.S. patent application number 13/203095 was filed with the patent office on 2011-12-15 for method and apparatus of precisely measuring intensity profile of x-ray nanobeam.
Invention is credited to Hidekazu Mimura, Hiromi Okada, Kazuto Yamauchi.
Application Number | 20110305317 13/203095 |
Document ID | / |
Family ID | 42665190 |
Filed Date | 2011-12-15 |
United States Patent
Application |
20110305317 |
Kind Code |
A1 |
Yamauchi; Kazuto ; et
al. |
December 15, 2011 |
METHOD AND APPARATUS OF PRECISELY MEASURING INTENSITY PROFILE OF
X-RAY NANOBEAM
Abstract
Provided are a method and an apparatus of precisely measuring
the intensity profile of an x-ray nanobeam, which can measure
x-rays having different wavelengths with one knife edge and can
perform optimal measurements corresponding to the depth of focus of
an x-ray beam and the conditions of other measurement devices,
using a dark field measurement method which enables precise
measurements of the profile of an x-ray beam using a knife edge and
using diffracted and transmitted x-rays. The knife edge (4) is
formed of a heavy metal which advances the phase of an x-ray
passing therethrough and is fabricated in such a manner that the
thickness may change in the longitudinal direction continuously or
in a stepwise fashion. The knife edge (4) is so set that an x-ray
beam may traverse the knife edge (4) at such a thickness position
as to achieve a phase shift in a range wherein a transmitted x-ray
and a diffracted x-ray diffracted at the end of the knife edge may
reinforce each other, and a superposed x-ray of the diffracted
x-ray and the transmitted x-ray is measured by an x-ray
detector.
Inventors: |
Yamauchi; Kazuto; (Osaka,
JP) ; Mimura; Hidekazu; (Osaka, JP) ; Okada;
Hiromi; (Hyogo, JP) |
Family ID: |
42665190 |
Appl. No.: |
13/203095 |
Filed: |
March 19, 2009 |
PCT Filed: |
March 19, 2009 |
PCT NO: |
PCT/JP2009/055474 |
371 Date: |
August 24, 2011 |
Current U.S.
Class: |
378/70 |
Current CPC
Class: |
G21K 2207/00 20130101;
G01N 23/207 20130101; G21K 2201/06 20130101; G01N 23/201
20130101 |
Class at
Publication: |
378/70 |
International
Class: |
G01N 23/207 20060101
G01N023/207 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 27, 2009 |
JP |
2009-045688 |
Claims
1. A method for precise measurement of an X-ray nanobeam intensity
distribution that uses a dark-field metrology to run a knife edge
so as to cut across an X-ray beam and measure an X-ray intensity by
an X-ray detector disposed behind the knife edge at a position
geometrically dark with respect to an X-ray source, thereby to
measure an X-ray intensity distribution in a cross section of the
X-ray beam, wherein the knife edge is made of a heavy metal with
the effect of advancing a phase of an X-ray passing through the
knife edge, prepared so as to change in thickness continuously or
stepwise in a longitudinal direction, and set so as to cut across
an X-ray beam at a position of a thickness as to obtain a phase
shift with which a transmission X-ray and a diffraction X-ray
diffracted by a leading end of the knife edge reinforce each other,
and an X-ray formed by overlapping of the diffraction X-ray and the
transmission X-ray is measured by the X-ray detector.
2. The method for precise measurement of an X-ray nanobeam
intensity distribution according to claim 1, wherein the knife edge
is formed so as to change in thickness from 1 .mu.m to 5 .mu.m
continuously or stepwise in a longitudinal direction and is set so
as to cut across an X-ray beam at a position of a thickness where a
transmission rate of an X-ray passing through the knife edge falls
within a range from 80% to 20% and a phase shift becomes 0.3.lamda.
to 0.7.lamda. (.lamda. denotes wavelength of an X-ray), and an
X-ray formed by overlapping of a diffraction X-ray that has been
diffracted at a leading end of the knife edge and come around
behind the knife edge and a transmission X-ray that has been passed
through the knife edge and advanced in phase, is measured by the
X-ray detector.
3. The method for precise measurement of an X-ray nanobeam
intensity distribution according to claim 1 or 2, wherein the
material for the knife edge is Pt or Au.
4. The method for precise measurement of an X-ray nanobeam
intensity distribution according to claim 1 or 2, wherein a leading
end portion of the knife edge is rectangular in cross section, and
a leading end surface of the knife edge has an inclination angle of
1 mrad or less.
5. The method for precise measurement of an X-ray nanobeam
intensity distribution according to claim 1 or 2, wherein the
leading end portion of the knife edge is rectangular in cross
section, and an angle formed by the leading end surface of the
knife edge and an optical axis of an X-ray beam is set at 1 mrad or
less.
6. The method for precise measurement of an X-ray nanobeam
intensity distribution according to claim 1 or 2, wherein an edge
member with the knife edge is run in a direction that the knife
edge cuts across an X-ray beam and in a direction along a longer
side of the knife edge.
7. An apparatus for precise measurement of an X-ray nanobeam
intensity distribution, comprising: an edge member that varies in
thickness continuously or stepwise in a longitudinal direction and
includes a knife edge with a leading end portion rectangular in
cross section and disposed such that an inclination angle of a
leading end surface becomes 1 mrad or less with respect to an
optical axis of an X-ray beam; a high-accurate moving stage that
holds the edge member such that the knife edge is run in a
direction that cuts across the X-ray beam and in a direction along
a longer side of the knife edge; and an X-ray detector that is
disposed behind the knife edge at a position geometrically dark
with respect to an X-ray source, wherein the knife edge is made of
a heavy metal with the effect of advancing a phase of an X-ray
passing through the knife edge and is set so as to cut across an
X-ray beam at a position of a thickness as to obtain a phase shift
with which a transmission X-ray and a diffraction X-ray diffracted
by a leading end of the knife edge reinforce each other, and an
X-ray formed by overlapping of the diffraction X-ray and the
transmission X-ray is measured by the X-ray detector.
8. The apparatus for precise measurement of an X-ray nanobeam
intensity distribution according to claim 7, wherein the knife edge
is formed so as to change in thickness from 1 .mu.m to 5 .mu.m
continuously or stepwise in a longitudinal direction and is set so
as to cut across an X-ray beam at a position of a thickness where a
transmission rate of an X-ray passing through the knife edge falls
within a range from 80% to 20% and a phase shift becomes 0.3.lamda.
to 0.7.lamda. (.lamda. denotes wavelength of an X-ray), and an
X-ray formed by overlapping of a diffraction X-ray that has been
diffracted at a leading end of the knife edge and come around
behind the knife edge and a transmission X-ray that has been passed
through the knife edge and advanced in phase, is measured by the
X-ray detector.
9. The apparatus for precise measurement of an X-ray nanobeam
intensity distribution according to claim 7 or 8, wherein a slit is
arranged in front of the X-ray detector such that an opening
thereof is situated at a position geometrically dark with respect
to an X-ray source.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method and apparatus for
precision measurement of an X-ray nanobeam intensity distribution,
and more specifically, to a method and apparatus for precision
measurement of an X-ray nanobeam intensity distribution that make
it possible to measure an intensity distribution of X-ray nanobeams
in soft X-ray to hard X-ray regions, with nm-order spatial
resolutions.
BACKGROUND ART
[0002] High-brightness, low-emittance, and high-coherence X-rays in
various wavelength regions from soft X-rays to hard X-rays have
become available at third-generation synchrotron radiation
facilities represented by SPring-8. This has dramatically improved
analytical sensitivities and spatial resolutions at various
analyses such as fluorescent X-ray analysis, photoelectron
spectrometry, and X-ray diffraction. These X-ray analyses and X-ray
microscopic approaches using radiation light not only provide high
sensitivities and high resolutions but also allow nondestructive
observations, and thus are currently being employed in the fields
of medicine, biology, and material science, and the like.
[0003] Highly collected X-ray nanobeams are required to utilize
various X-ray analytical technologies with high spatial resolutions
at synchrotron radiation facilities. A group of the inventors has
already succeeded in collecting an X-ray with a wavelength of 0.6
.ANG. in a spot diameter of 100 nm or less, by using a light
collection optical system including a Kirkpatrick and Baez (K-B)
mirror. This success is largely due to a uniquely developed
high-precision mirror processing technique and high-precision
mirror shape measurement techniques. This processing technique
refers to numerically-controlled elastic emission machining (EEM)
which is performed on a process principle: a high shear flow of
ultrapure water mixed with fine particles is formed along a surface
of a mirror to be processed; the fine particles combine together
with atoms on the surface of the mirror by a kind of chemical
reaction; and the surface atoms are removed with movement of the
fine particles. In addition, the shape measurement technologies
refer to microstitching interferometry (MSI) and relative angle
determinable stitching interferometry (RADSI) which are performed
on a measurement principle that pieces of partial shape data from
an interferometer capable of high-precision shape measurement of
small areas are put together to obtain the entire shape data. Using
the shape measurement techniques makes it possible to measure
accurately the shape of an X-ray mirror in all space wavelength
ranges with a measurement reproducibility of 1 nm or less of PV
value. The group has successfully prepared an X-ray light
collecting mirror with an accuracy of 2 nm (PV value) using these
techniques, thereby to realize diffraction-limited light collection
of SPring-8 hard X-rays at a level of sub-30 nm.
[0004] The inventors aim to realize sub-10 nm light collection for
implementation of the world's best ultrahigh-resolution scanning
X-ray microscope and ultrahigh-resolution X-ray micro CT. To that
end, extremely strict shape accuracy is required for X-ray mirrors
as follows: a shape error is P-V1 nm or less in mid- and long-term
space wavelengths; a designed mirror shape has a deep curve; a
multilayer film is essentially formed on a mirror surface to
provide a deep X-ray incident angle, and the like. Accordingly, it
is extremely difficult to determine a phase error in a surface of
an X-ray mirror with respect to an ideal surface by off-line
measurement using an interferometer or the like. The inventors
therefore have proposed an at-wavelength metrology in which a phase
error in a mirror surface is determined by phase retrieval
calculation only from X-ray intensity profile information in a
light collection plane, and proposed an X-ray collection method in
which a phase error of a light collection optical system is
corrected using the foregoing metrology to eliminate irregularities
in a wavefront of a focal plane (JP 2006-357566 (JP 2008-164553
A)). To calculate precisely a phase error of an X-ray mirror by the
phase retrieval method, it is essentially required to acquire an
accurate X-ray collection intensity profile.
[0005] Conventionally, an X-ray beam intensity profile is measured
in such a manner as to cut off an X-ray beam little by little by a
knife edge or a wire while measuring changes in light intensity as
described in Patent Document 1. FIG. 14 shows a measurement optical
system using a wire scanning method. In this optical system, an
incident X-ray 100 is passed through a slit 101 so as to be limited
to a predetermined width, then is passed through an ion chamber
102, and then is reflected and collected by a surface of an X-ray
mirror 103. In the foregoing arrangement, an Au wire 104 with a
diameter of 200 .mu.m sufficiently larger than a diameter of an
X-ray beam is run by a piezo stage in a light collection plane
vertically to the mirror surface, thereby to gradually cut off a
collected beam while measuring changes in X-ray intensity behind
the focal point through the slit 105 by an X-ray detector 106. In
this arrangement, as the X-ray detector 106, an avalanche
photodiode (APD) with high sensitivity and fast output
responsibility is used. The X-ray intensities measured by the X-ray
detector 106 are standardized in accordance with an incident X-ray
intensity measured at the ion chamber 102. The slit 105 is provided
to eliminate influence of inclination of the wire 104 with respect
to the beam on measurement of a light collection intensity profile.
FIG. 15 (a) shows changes in X-ray intensity profile measured by
the X-ray detector 106. These changes are differentiated with
respect to wire positions, thereby to obtain a light collection
intensity profile as shown in FIG. 15 (b).
[0006] However, the wire scanning method has two problems: it is
difficult to prepare a geometrically sharp knife edge with a
sufficient thickness so as not to let an X-ray pass through; and
noise generated at intensity measurement is enhanced at the time of
differentiation. In addition, although accurate information is
needed in a wide base region of an X-ray intensity profile to
calculate precisely a phase error of an X-ray mirror by phase
retrieval, the conventional wire scanning method provides
information in this region with low reliability.
[0007] Accordingly, in order to provide a method and apparatus for
precise measurement of an X-ray nanobeam intensity distribution
that overcome the problem of noise enhancement due to background
noise and differentiation associated with the wire scanning method
and realize higher-precision X-ray beam profile measurement, the
inventors propose a method for precise measurement of an X-ray
nanobeam intensity distribution that use a dark-field metrology to
run a knife edge so as to cut across an X-ray beam and measure an
X-ray intensity by an X-ray detector disposed behind the knife edge
at a position geometrically dark with respect to an X-ray source,
thereby to measure an X-ray intensity distribution in a cross
section of the X-ray beam, wherein the knife edge is made of a
heavy metal with the effect of advancing a phase of an X-ray
passing through the knife edge, a thickness of the knife edge is
set so as to obtain a phase shift to an extent that the
transmission X-ray and a diffraction X-ray diffracted by a leading
end of the knife edge reinforce each other, and an X-ray formed by
overlapping of the diffraction X-ray and the transmission X-ray is
measured by the X-ray detector. [0008] Patent Document 1: JP-A No.
10-319196
SUMMARY OF INVENTION
Technical Problem
[0009] In the foregoing measurement method proposed by the
inventors, however, a knife edge of a theoretically optimum
thickness is used for an X-ray of a specific wavelength, which
means that knife edges of different thicknesses are needed for
X-rays of different wavelengths. This causes troublesome
replacement tasks of knife edges and requires uneconomically a
large number of expensive knife edges. In addition, setting the
thickness of a knife edge at a theoretically optimum value may not
realize optimum intensity measurement, depending on a focal depth
of an X-ray beam and other conditions of the measurement
apparatus.
[0010] In light of the foregoing circumstances, an object of the
present invention is to provide a method and apparatus for precise
measurement of an X-ray nanobeam intensity distribution that use a
dark-field metrology allowing high-precision measurement of an
X-ray beam profile using a knife edge, a diffraction X-ray, and a
transmission X-ray, support measurement of X-rays of different
wavelengths with one knife edge, and realize optimum measurement in
accordance with a focal depth of an X-ray beam and other conditions
of the measurement apparatus.
Solution to Problem
[0011] To solve the foregoing problem, the present invention
provides a method for precise measurement of an X-ray nanobeam
intensity distribution that uses a dark-field metrology to run a
knife edge so as to cut across an X-ray beam and measure an X-ray
intensity by an X-ray detector disposed behind the knife edge at a
position geometrically dark with respect to an X-ray source,
thereby to measure an X-ray intensity distribution in a cross
section of the X-ray beam, wherein the knife edge is made of a
heavy metal with the effect of advancing a phase of an X-ray
passing through the knife edge, prepared so as to change in
thickness continuously or stepwise in a longitudinal direction, and
set so as to cut across an X-ray beam at a position of a thickness
as to obtain a phase shift with which a transmission X-ray and a
diffraction X-ray diffracted by a leading end of the knife edge
reinforce each other, and an X-ray formed by overlapping of the
diffraction X-ray and the transmission X-ray is measured by the
X-ray detector (Claim 1).
[0012] In addition, preferably, the knife edge is formed so as to
change in thickness from 1 to 5 .mu.m continuously or stepwise in a
longitudinal direction and is set so as to cut across an X-ray beam
at a position of a thickness where a transmission rate of an X-ray
passing through the knife edge falls within a range from 80 to 20%
and a phase shift becomes 0.3 to 0.7.lamda. (.lamda. denotes
wavelength of an X-ray), and an X-ray formed by overlapping of a
diffraction X-ray that has been diffracted at a leading end of the
knife edge and come around behind the knife edge and a transmission
X-ray that has been passed through the knife edge and advanced in
phase, is measured by the X-ray detector (Claim 2).
[0013] In this arrangement, the material for the knife edge is
preferably Pt or Au (Claim 3). More preferably, a leading end
portion of the knife edge is rectangular in cross section, and a
leading end surface of the knife edge has an inclination angle of 1
mrad or less (Claim 4); or the leading end portion of the knife
edge is rectangular in cross section, and an angle formed by the
leading end surface of the knife edge and an optical axis of an
X-ray beam is set at 1 mrad or less (Claim 5).
[0014] Further preferably, an edge member with the knife edge is
run in a direction that the knife edge cuts across an X-ray beam
and in a direction along a longer side of the knife edge (Claim
6).
[0015] In addition, for solving the foregoing problem, the present
invention provides an apparatus for precise measurement of an X-ray
nanobeam intensity distribution, comprising: an edge member that
varies in thickness continuously or stepwise in a longitudinal
direction and includes a knife edge with a leading end portion
rectangular in cross section and disposed such that an inclination
angle of a leading end surface becomes 1 mrad or less with respect
to an optical axis of an X-ray beam; a high-accurate moving stage
that holds the edge member such that the knife edge is run in a
direction that cuts across the X-ray beam and in a direction along
a longer side of the knife edge; and an X-ray detector that is
disposed behind the knife edge at a position geometrically dark
with respect to an X-ray source, wherein the knife edge is made of
a heavy metal with the effect of advancing a phase of an X-ray
passing through the knife edge and is set so as to cut across an
X-ray beam at a position of a thickness as to obtain a phase shift
with which a transmission X-ray and a diffraction X-ray diffracted
by a leading end of the knife edge reinforce each other, and an
X-ray formed by overlapping of the diffraction X-ray and the
transmission X-ray is measured by the X-ray detector (Claim 7).
[0016] Further, preferably, the knife edge is formed so as to
change in thickness from 1 to 5 .mu.m continuously or stepwise in a
longitudinal direction, and is set so as to cut across an X-ray
beam at a position of a thickness where a transmission rate of an
X-ray passing through the knife edge falls within a range from 80
to 20% and a phase shift becomes 0.3 to 0.7.lamda. (.lamda. denotes
wavelength of an X-ray), and an X-ray formed by overlapping of a
diffraction X-ray that has been diffracted at a leading end of the
knife edge and come around behind the knife edge and a transmission
X-ray that has been passed through the knife edge and advanced in
phase, is measured by the X-ray detector (Claim 8).
[0017] More preferably, a slit is arranged in front of the X-ray
detector such that an opening thereof is situated at a position
geometrically dark with respect to an X-ray source (Claim 9).
Advantageous Effects of Invention
[0018] According to a method and apparatus for precise measurement
of an X-ray nanobeam intensity distribution in the present
invention, a diffraction X-ray intensity can be directly detected
in proportion to an X-ray intensity at the leading end position of
a knife edge in a geometrically dark section, which eliminates the
need for differential processing required in the conventional wire
scanning method and thus allows measurement with low background
noise. In addition, the knife edge is made of a heavy metal with
the effect of advancing a phase of an X-ray passing through the
knife edge, a thickness of the knife edge is set so as to obtain a
phase shift with which a transmission X-ray and a diffraction X-ray
diffracted at the leading end of the knife edge reinforce each
other, and an X-ray formed by overlapping of the diffraction X-ray
and the transmission X-ray is measured by an X-ray detector. This
enhances a signal level, which increases an S/N ratio allowing
measurement of an X-ray intensity distribution with high
sensitivity and high spatial resolution. In particular, it is
possible to measure an intensity distribution of an X-ray nanobeam
collected with a full width at half maximum of a beam waist of 100
nm or less, with nm-order spatial resolutions.
[0019] In addition, the thickness of the knife edge is changed
continuously or stepwise with respect to the optical axis of an
X-ray, and the edge member with the knife edge is run in a
direction that changes in thickness and is orthogonal to the
direction of the optical axis, thereby allowing the knife edge to
be optimum in thickness with respect to the wavelength of an X-ray.
If a focal depth of an X-ray is shallow, although sensitivity is
sacrificed, a thinner portion of the knife edge can be used to
obtain a sharp profile. Further, if the wavelength of an X-ray is
unknown, it is possible to obtain a wavelength range of the unknown
X-ray by deriving the thickness of the knife edge with a maximum
diffraction X-ray intensity with respect to the X-ray of an unknown
wavelength, or deriving thickness-intensity measurement
characteristics from measurement of changes in diffraction X-ray
intensity with respect to changes in thickness of the knife edge,
and determining the wavelength range by back calculation from
comparison between the thickness-intensity measurement
characteristics and thickness-intensity calculation characteristics
obtained by calculating the diffraction X-ray intensity with
respect to the wavelength of the X-ray and the thickness of the
knife edge.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a general arrangement diagram of a measurement
optical system for realizing a method for precise measurement of an
X-ray nanobeam intensity distribution in the present invention;
[0021] FIG. 2 is an illustrative diagram showing a sub-30 nm light
collection optical system used for measurement of an X-ray beam
intensity distribution;
[0022] FIG. 3 (a) is a graph showing a designed mirror shape of the
sub-30 nm light collection optical system, and FIG. 3 (b) is a
graph showing an ideal light collection profile;
[0023] FIG. 4 (a) is an arrangement diagram showing a relationship
between a knife edge and an X-ray beam, and FIG. 4 (b) is a graph
showing relationships among an intensity of a transmission X-ray, a
phase shift of the transmission X-ray, and an intensity of a
diffraction X-ray, with changes in thickness of the knife edge;
[0024] FIG. 5 (a) is an illustrative diagram showing a positional
relationship between a leading end shape of the knife edge and an
X-ray beam, and FIG. 5 (b) is a graph showing calculation results
of a light collection profile with changes in inclination of the
leading end of the knife edge;
[0025] FIG. 6 show a method for manufacturing the knife edge in the
present invention: FIG. 6 (a) illustrates a base cut away from an
Si wafer; FIG. 6 (b) illustrates the base on which Pt is
evaporated; and FIG. 6 (c) illustrates formation of the knife edge
with a predetermined thickness by carving out of a periphery of the
base by an FIB processing device;
[0026] FIG. 7 is a perpendicular view of an outer appearance of an
edge member with the knife edge used in the present invention;
[0027] FIG. 8 is a graph of an X-ray beam intensity distribution
measured by the present invention;
[0028] FIG. 9 is graphs of X-ray beam intensity distributions
measured on a focal plane and at front and back sides of the
same;
[0029] FIG. 10 is a graph showing a shape error distribution of an
X-ray mirror calculated by a phase retrieval method only from the
X-ray intensity distribution of FIG. 8 and a shape error
distribution obtained by off-line measurement (RADSI) using an
interferometer;
[0030] FIG. 11 show the edge member with the knife edge used in the
present invention: FIG. 11 (a) is a perpendicular view of an outer
appearance of the edge member; and FIG. 11 (b) is a plane view of
the edge member;
[0031] FIG. 12 show embodiments of the knife edge: FIG. 12(a) is a
partial plane view of a knife edge of a two-sided inclination type
which has on both sides inclination planes symmetric with respect
to a center line and changes in thickness in a continuous manner;
FIG. 12(b) is a partial plane view of a knife edge of a
single-sided inclination type which has an inclination plane on one
side and an orthogonal plane on the other side, and changes in
thickness in a continuous manner; FIG. 12(c) is a partial plane
view of a knife edge of a two-sided stepped type which has on both
sides stepped planes symmetric with respect to a center line, and
changes in thickness in a stepwise manner; and FIG. 12(d) is a
partial plane view of a knife edge of a single-sided stepped type
which has a stepped plane on one side and an orthogonal plane on
the other side, and changes in thickness in a stepwise manner;
[0032] FIG. 13 is a simplified perpendicular diagram showing a
relationship between an optical axis of an X-ray and a running
direction of the edge member;
[0033] FIG. 14 is a general arrangement diagram of a measurement
optical system using a conventional wire scanning method; and
[0034] FIG. 15 show results of measurement by the conventional wire
scanning method: FIG. 15 (a) is a graph showing changes in X-ray
intensity with displacement of a wire; and FIG. 15 (b) is a graph
showing an X-ray intensity profile obtained by differentiating the
results (a).
DESCRIPTION OF EMBODIMENTS
[0035] The present invention will be described in more detail with
reference to the attached drawings. FIG. 1 is a general arrangement
diagram of a measurement optical system using a method for precise
measurement of an X-ray nanobeam intensity distribution, and FIG. 2
shows an X-ray beam collection optical system used for
measurement.
[0036] In this embodiment, as shown in FIG. 1, an incident X-ray 1
passes through a slit 2 and obliquely enters into an X-ray mirror 3
having an oval form where the incident X-ray 1 is subjected to
one-dimensional light collection. In addition, a knife edge 4 is
disposed on an X-ray beam focal plane and a slit 5 is arranged
behind the knife edge 4 to shut off a direct X-ray beam, and an
X-ray intensity is measured by an X-ray detector 6 that is disposed
behind the knife edge 4 at a position geometrically dark with
respect to an X-ray source. The knife edge 4 is held by a moving
stage 7, and the moving stage 7 is driven to run the knife edge 4
so as to cut across the X-ray beam. In this embodiment, the moving
stage 7 is configured as a piezo-stage to provide a running
accuracy of 1 nm. In addition, the moving stage 7 is configured to
move the knife edge 4 in a direction of an optical axis of an X-ray
beam and adjust an angle of inclination of the knife edge 4 with
respect to an X-ray beam.
[0037] In this arrangement, the X-ray detector 6 uses an avalanche
photodiode (APD) with high sensitivity and fast output
responsibility. In addition, for standardization of an X-ray
intensity measured by the X-ray detector 6, an ion chamber 8 is
disposed immediately in front of the X-ray mirror 3 to thereby
measure an incident X-ray intensity at any time.
[0038] The X-ray beam used in this embodiment is a SPring-8 1
km-long beam line (BL29XUL) with X-ray energy of 15 keV (wavelength
.lamda.=0.8 .ANG.). FIGS. 2 and 3 show characteristics of an X-ray
beam collection optical system. As shown in FIG. 2, the X-ray beam
passes through a 10 .mu.m-wide slit, and then is collected by an
X-ray mirror 1 km ahead, at a position at a focal distance of 150
mm. As shown in FIG. 3 (a), the X-ray mirror has a reflection plane
designed to have an oval shape 100 mm long and about 10 .mu.m deep
in a central portion. In addition, the reflection plane of the
X-ray mirror has a shape accuracy of 2 nm (PV value) or less. FIG.
3 (b) shows an ideal light collection profile with the thus
designed X-ray mirror. If light collection is performed by an ideal
X-ray mirror, a full width at half maximum (FWHM) of a beam waist
is about 25 nm. The oval X-ray collection mirror utilizes
geometrical nature of an oval to preserve a wavefront by
maintaining at a constant level an X-ray overall optical path
length from a light source to a focal point, and obtain ideal light
collection with complete phase matching at the focal point.
[0039] A method for precise measurement of an X-ray nanobeam
intensity distribution in the present invention uses a dark-field
metrology to run a knife edge so as to cut across an X-ray beam and
measure an X-ray intensity by an X-ray detector disposed behind the
knife edge at a position geometrically dark with respect to an
X-ray source, thereby to measure an X-ray intensity distribution in
a cross section of the X-ray beam, and the method is characterized
in that the knife edge is made of a heavy metal with the effect of
advancing a phase of an X-ray passing through the knife edge, a
thickness of the knife edge is set so as to obtain a phase shift to
an extent that the transmission X-ray and a diffraction X-ray
diffracted by a leading end of the knife edge reinforce each other,
and an X-ray formed by overlapping of the diffraction X-ray and the
transmission X-ray is measured by the X-ray detector.
[0040] Measurement principle of the present invention will be
briefly described below. When a leading edge portion of the knife
edge is positioned in an X-ray beam formed by a planar wave, a
phenomenon (diffraction) occurs that a spherical wave is generated
at the edge portion and the X-ray comes around behind the knife
edge. In addition, part of the X-ray passes through the leading
edge portion of the knife edge. If the material for the knife edge
has the effect of advancing a phase of the X-ray passing through
the knife edge, the phase of the transmission X-ray shifts
depending on the thickness of the knife edge and the transmission
X-ray decreases in intensity. Then, the diffraction X-ray and the
transmission X-ray overlap behind the leading edge portion of the
knife edge. If the phase shift of the transmission X-ray occurs
only by a half-wavelength with a sufficient transmission intensity
maintained, the transmission X-ray and the diffraction X-ray
reinforce each other at the time of overlapping. The inventors have
discovered from results of simulations that an X-ray having reached
behind the knife edge has an intensity in proportion to the X-ray
beam intensity at the edge portion. Accordingly, measuring the
intensity of this X-ray at a position geometrically dark with
respect to the X-ray beam, allows direct measurement of an
intensity profile of the X-ray beam without influence of background
noise. In addition, without the need to differentiate measured
values as in the conventional wire scanning method, the measurement
method of the present invention makes it possible to avoid
enhancement of noise and minimize influence of noise, thereby to
realize high-sensitivity, high-precision measurement.
[0041] In addition, the X-ray detector is disposed at a position
that does not detect directly the transmission X-ray having passed
through the knife edge 4. Alternatively, the slit 5 disposed in
front of the X-ray detector 6 shut off the X-ray. In addition, the
X-ray detector 6 is arranged at a position distant as much as
possible from a geometrical light path of the X-ray beam for
detection of intensities of the diffraction X-ray and the
transmission X-ray. In this arrangement, positional accuracies
required for the X-ray detector 6 and the slit 5 are lower because
the diffraction X-ray does not greatly change in intensity even if
the position of the X-ray detector 6 is displaced by 3 to 5 mm.
Regarding this point, the inventors have verified from simulations
that positional dependence of the diffraction X-ray intensity on
the X-ray detector 6 is extremely low in a geometrically dark
section.
[0042] In this arrangement, a typical heavy metal with the effect
of advancing a phase of a transmission X-ray is Pt or Au.
Alternatively, other heavy metals may be used for an optimum
designed thickness in accordance with a wavelength and a focal
depth of an X-ray and a required spatial resolution. Although the
X-ray beam handled in this embodiment has energy of 10 to 20 keV
(with a wavelength of 1.2 to 0.6 .ANG.), it is also possible to
measure an intensity distribution of X-ray beams in a wider range
of wavelengths. Since an X-ray of a longer wavelength has a larger
amount of phase shift, measurement with higher spatial resolution
is allowed using a thinner knife edge. Further, there is a
possibility that the technique of the present invention can be
employed to measure an intensity distribution of an extreme
ultraviolet ray of a wavelength of 13.5 nm used for extreme ultra
violet lithography (EUVL) as a next-generation semiconductor
exposure technology.
[0043] Next, the inventors have performed simulations using Pt as
the material for the knife edge, and estimated an optimum thickness
of the knife edge for measurement of a light collection intensity
profile of an X-ray with a wavelength of 0.8 .ANG. collected by an
oblique incident optical system, and a shape accuracy of the
leading edge portion of the knife edge. The estimated results will
be described with reference to FIGS. 4 and 5. As shown in FIG. 4
(a), the knife edge is placed with the thickness oriented in the
direction of the optical axis and with the edge portion positioned
in a center of an X-ray beam. In this state, as shown in FIG. 4
(b), an intensity of a transmission X-ray (solid line), a phase
shift of the transmission X-ray (chain line), an intensity of a
diffraction X-ray (dotted line) were calculated with changes in
thickness of the knife edge. With increase in thickness of the
knife edge, the phase shift of the transmission X-ray increases
linearly but the intensity of the transmission X-ray decreases
exponentially. Therefore, the intensity of the diffraction X-ray
does not always become highest when the phase shift of the
transmission X-ray takes places by a half wavelength. Practically,
the thickness of the knife edge may be set such that the intensity
of the diffraction X-ray falls within a range covering about 80% of
the maximum value. Nevertheless, the knife edge is preferably
thinner as much as possible within an allowable range because the
thinner knife edge provides a higher spatial resolution. From the
foregoing results, in this embodiment, the Pt knife edge with a
thickness of 2,000 nm (2 .mu.m) is used for an X-ray beam with a
wavelength of 0.8 .ANG..
[0044] In addition, the inventors have estimated a required shape
accuracy of the leading edge portion of the knife edge by
calculating an intensity profile with variations in x on the basis
of a model shown in FIG. 5 (a). Specifically, the inventors have
added a right-triangular portion to a leading end surface of a
2,000-nm thick knife edge rectangular in cross section, and
calculated an intensity of a diffraction X-ray on the knife edge
while changing an inclination angle of the leading end surface with
variations in x of 0 nm, 2 nm, 5 nm, and 10 nm as shown in the
drawing. FIG. 5 (b) shows results of the calculation. The intensity
profile with x of 0 nm (shown by open circles) corresponds to an
ideal light collection profile. The intensity profile with x of 2
nm is plotted by rhombuses, the intensity profile with x of 5 nm by
squares, the intensity profile with x of 10 nm by triangles.
[0045] It is understood from the foregoing results that the
intensity profile with x of 2 nm has small and allowable deviations
from the ideal light collection profile, but the intensity profile
with x of 5 nm has too large deviations from the ideal light
collection profile. Therefore, the knife edge needs to be produced
such that the inclination angle of the leading end surface becomes
1 mrad or less. In addition, even if the leading end portion of the
knife edge is accurately produced so as to be rectangular in cross
section, when the leading end surface of the knife edge held by the
moving stage 7 inclines with respect to the optical axis of the
X-ray beam, the intensity of the diffraction X-ray also deviates
from the ideal light collection profile. Accordingly, it is
necessary to set an angle formed by the leading end surface of the
knife edge and the optical axis of the X-ray beam at 1 mrad or less
as described above. Therefore, the moving stage 7 is structured so
as to allow the posture of the knife edge 4 to be arbitrarily
fine-tuned.
[0046] The oblique incident light collection optical system with an
oval X-ray collection mirror has a deep focal depth, and thus
realizes measurement with higher spatial resolutions even if a
significantly thicker knife edge is used as compared with a full
width at half maximum of an X-ray beam. That is, even if an X-ray
beam is collected such that a beam waist becomes about 10 nm, it is
possible to use a 2,000 nm-thick knife edge rectangular in the
shape of a leading edge portion to measure an X-ray intensity
profile accurately with nm-order spatial resolutions.
[0047] Considering the foregoing results together, the present
invention is designed to set the thickness of the knife edge such
that a transmission rate of an X-ray passing through the knife edge
falls within a range of 80 to 20% and the phase shift of the X-ray
becomes 0.3 to 0.7.lamda.(.lamda. denotes a wavelength of the
X-ray), and measure by the X-ray detector an X-ray formed by
overlapping of a diffracted X-ray that has diffracted at the
leading end of the knife edge and come around behind the knife edge
and a transmission X-ray that has passed through the knife edge and
advanced in phase. Preferably, the thickness of the knife edge is
set such that a transmission rate of an X-ray passing through the
knife edge falls within a range of 80 to 20% and the phase shift of
the X-ray becomes 0.4 to 0.6.lamda..
[0048] Next, a method for manufacturing the knife edge will be
described below with reference to FIG. 6. From the foregoing
simulation results, the knife edge is to be made of Pt with a
thickness of 2,000 nm, a height of 0.5 .mu.m or more, and a width
of 50 p.m. First, an Si wafer is cut into a rectangle 0.9.times.9
mm (0.5 mm thick) to prepare a base 11 (see FIG. 6(a)). Next, Pt is
evaporated by electron beams on a surface of the base 11 to form a
Pt layer 12 with a thickness of 2 .mu.m (see FIG. 6(b)), and
finally a knife edge 13 with a thickness of 2 .mu.m is carved out
by FIB processing (see FIG. 6(c)). FIG. 7 shows schematically an
entire shape of the edge member 10 with the knife edge 13 formed.
In actuality, the base 11 of the edge member 10 is attached to the
moving stage 7.
[0049] The inventors have used the knife edge as specified above in
the measurement optical system of FIG. 1 to measure an X-ray
intensity profile of the X-ray beam in the ideal light collection
profile of FIG. 3 (b) on the focal plane. FIG. 8 shows results of
the measurement. It is understood from the results that a full
width at half maximum of beam waist of the X-ray beam has become
slightly larger than 25 nm in the ideal light collection profile,
but wave properties have been reproduced in a broad base region.
Accordingly, the measurement method in the present invention
obviously holds superiority, as compared with the results of
measurement by the conventional wire scanning method shown in FIG.
15 (b).
[0050] FIG. 9 is graphs of X-ray beam intensity distributions
measured on the focal plane (Y=0 .mu.m) and at front and back
positions of the same (Y=.+-.50 .mu.m). As in the drawing, the
measurement method of the present invention allows precise
measurement of an X-ray intensity distribution not only on the
focal plane but also at positions distant from the focal plane.
This allows not only determination of a spot diameter but also
analysis of a fine structure of a beam waist, thereby resulting in
improvement in quality of light collection. In addition, the
present invention also makes it possible to measure X-ray intensity
profiles by running the knife edge across an X-ray beam from a
plurality of directions and combine the measurements into a
three-dimensional intensity profile.
[0051] An X-ray may be distorted in wavefront at reflection on an
X-ray mirror under influence of shape error of the X-ray mirror and
thickness error of a multilayer film on the X-ray mirror. Such
influence affects differently an intensity profile of an X-ray beam
actually measured on the focal plane, depending on magnitude of the
shape error and space wavelength. In such cases, the distorted
light collection profile is considered to include information on
the shape error of the X-ray mirror. Therefore, the phase error of
the X-ray mirror can be calculated by a phase retrieval method from
the X-ray intensity profile on the focal plane or in the vicinity
of the same (see JP 2006-357566 A). Since the influence of the
shape error of the X-ray mirror appears in a broad base region of
the X-ray intensity profile of the X-ray beam measured on the focal
plane or in the vicinity of the same, it is important to measure
precisely an X-ray intensity profile covering a broad base region
for accurate calculation of a shape error of the X-ray mirror.
[0052] The inventors have calculated a shape error of the X-ray
mirror by the phase retrieval method using the measurement results
of the intensity profile of the X-ray beam shown in FIG. 8. FIG. 10
shows calculation results by a bold solid line (in low cycles).
FIG. 10 also provides results of off-line measurement using an
interferometer by a narrow solid line (in high cycles). The two
results exhibit an extremely favorable concordance, which proves
high effectiveness and reliability of the measurement method of the
present invention. However, even if the thickness of the knife edge
is set at a theoretically optimum value, optimum measurement may
not be obtained depending on the focal depth of an X-ray beam and
other conditions of a measurement apparatus.
[0053] Accordingly, the inventors propose a method and apparatus
for precise measurement of an X-ray nanobeam intensity distribution
using a knife edge varied in thickness continuously or stepwise in
a longitudinal direction, as shown in FIGS. 11 to 13. The edge
member 10 used in this embodiment has a knife edge 14 formed so as
to vary in thickness continuously in a longitudinal direction, as
shown in FIG. 11. The knife edge 14 is configured to have a length
of 200 .mu.m, a minimum thickness of 1 .mu.m, and a maximum
thickness of 5 .mu.m and vary in thickness linearly between the
minimum and maximum portions. In this arrangement, if the length of
the knife edge 14 is about 200 .mu.m and a diameter of an X-ray
beam (FWHM) is 100 nm or less, the thickness of the knife edge 14
can be regarded as approximately constant within the range of the
beam diameter, and does not arise any problem in measurement of an
X-ray intensity profile. In addition, if the minimum thickness of
the knife edge 14 is thinner than 1 .mu.m, the knife edge 14 cannot
provide a sufficient amount of phase shift and cannot be readily
handled due to its weakened mechanical strength. Meanwhile, if the
maximum thickness of the knife edge 14 is larger than 5 .mu.m,
transmission attenuation of an X-ray becomes too large to utilize
the measurement principle of the present invention using a
transmission X-ray, thereby resulting in a deteriorated S/N
ratio.
[0054] FIG. 12 illustrates various shapes of knife edges 14A, 14B,
14C, and 14D with thickness varied in a longitudinal direction. The
knife edge 14A shown in FIG. 12(a) is identical to that shown in
FIG. 11, but is a tapered two-sided inclination type in which
inclined surfaces 15, 15 are formed on both sides so as to be
symmetric with respect to a center line, and is continuously
changed in thickness in a longitudinal direction. In this
arrangement, the inclined surfaces 15 incline with respect to a
flat plane orthogonal to the optical axis of an X-ray. The knife
edge 14B shown in FIG. 12(b) is a single-sided inclination type in
which the inclined surface 15 is formed on one side and an
orthogonal surface 16 is formed on the other side, and is
continuously changed in thickness in a longitudinal direction. In
this arrangement, the orthogonal surface 16 refers to a flat plane
orthogonal to the optical axis of an X-ray. The knife edge 14C
shown in FIG. 12(c) is a two-sided stepped type in which stepped
surfaces 17, 17 are formed on both sides so as to be symmetric with
respect to a center line, and is changed stepwise in thickness in a
longitudinal direction. The knife edge 14D shown in FIG. 12(d) is a
single-sided stepped type in which the stepped surface 17 is formed
on one side and the orthogonal surface 16 is formed on the other
side, and is changed stepwise in thickness in a longitudinal
direction. In any of the foregoing types, the thickness of the
knife edge 14 is varied from 1 to 5 .mu.m. In the stepped types,
the stepped surfaces 17 are each configured by a flat plane
orthogonal to the optical axis of an X-ray. Alternatively, the
knife edge 14 may be configured so as to be the thinnest in a
middle portion and be made thicker on the both sides thereof in a
symmetrical manner.
[0055] Then, the edge member 10 with the knife edge 14 formed so as
to vary in thickness continuously or stepwise in a longitudinal
direction, is fixed to the moving stage 7. As shown in FIG. 13, the
moving stage 7 is configured to run the knife edge 14 with nm-order
accuracy in two directions orthogonal to the optical axis of an
X-ray (V and H directions). In this arrangement, the V direction
corresponds to a vertical direction with the base 11 of the edge
member 10 horizontally disposed, along the width of the knife edge
14. The H direction corresponds to a horizontal direction with the
base 11 of the edge member 10 horizontally disposed, along the
length of the knife edge 14. For measurement of an X-ray intensity
profile, first, if the wavelength of an X-ray is known, the edge
member 10 is run in the H direction to set the knife edge 14 so as
to cut across an X-ray beam at a portion with a thickness of a
theoretically optimum value. In this arrangement, since the
thickness of the knife edge 14 corresponds one-on-one to a
coordinate in the H direction, the thickness of the knife edge 14
can be properly set by monitoring the coordinate in the H
direction. Then, the edge member 10 is run in the V direction such
that the knife edge 14 cuts across the X-ray beam as described
above, thereby measuring an X-ray intensity profile.
[0056] In addition, an X-ray intensity profile can be measured at
maximum sensitivity in such a manner as to: run the edge member 10
in the H direction when part of an X-ray beam contacts the leading
end portion of the knife edge 14; measure a diffraction X-ray
intensity with respect to the thickness of the knife edge 14 to
obtain the thickness-intensity measurement characteristics M as
shown in FIG. 4 (b); specify the thickness of the knife edge 14
with maximum intensity of the X-ray beam; and run the edge member
10 in the H direction to a position with the specified thickness of
the knife edge 14, and then run the edge member 10 in the V
direction at the position with the specified thickness of the knife
edge 14. Even if the wavelength of an X-ray is unknown, a
wavelength range of the unknown X-ray can be determined by inverse
calculation from comparison between the thickness-intensity
measurement characteristics M and thickness-intensity calculation
characteristics S obtained by calculating a diffraction X-ray
intensity with respect to the wavelength of the X-ray and the
thickness of the knife edge. In addition, it is possible to store
table data of the thickness-intensity calculation characteristics S
with respect to wavelengths of a large number of X-rays in advance
in a memory of an X-ray measurement apparatus, thereby to perform
promptly data processing at the running in the H direction, which
is suitable for measurement of an X-ray intensity profile in real
time.
REFERENCE SIGNS LIST
[0057] 1. Incident X-ray [0058] 2. Slit [0059] 3. X-ray mirror
[0060] 4. Knife edge [0061] 5. Slit [0062] 6. X-ray detector [0063]
7. Moving stage [0064] 8. Ion chamber [0065] 10 Edge member [0066]
11. Base [0067] 12 Pt layer [0068] 13 Knife edge [0069] 14, 14A,
14B, 14C, and 14D Knife edge [0070] 100 Incident X-ray [0071] 101
Slit [0072] 102 Ion chamber [0073] 103 X-ray mirror [0074] 104 Au
wire [0075] 105 Slit [0076] 106 X-ray detector
* * * * *